Wash the spinach well in several changes of cold water, spin dry, then bundle in paper towels and refrigerate. When ready to proceed, mound spinach in a large, heatproof salad bowl.

Brown the bacon in a large, heavy skillet over moderate heat for 3 to 5 minutes until golden and crispy. Drain crisp bacon bits on paper towels and set aside.

Drain off all but 2 tablespoons bacon drippings. Stir in the vinegar, scraping up all cooked-on bits of bacon. Whisk in the brown sugar, salt and black pepper. Add the olive oil, then adjust seasonings; set dressing aside while you assemble salad ingredients.

Tear spinach into bite-sized pieces, discarding tough stems. For individual servings, divide spinach between 6 to 8 salad bowls or plates. Over each bowl, layer the mushrooms, hazelnuts and some dressing. Sprinkle each serving with some of the egg, crumbled bacon and Parmesan cheese. For 1 large bowl, prepare as above and toss salad at the table right before serving.